NO. II DIGESTIVE CANAL OF AMERICAN ALLIGATOR REESE 1/ 



or cuboidal cells with large nuclei. On tlie ventral side, where the 

 oesophageal wall is in contact with that of the trachea the epithelium 

 is somewhat thickened by an increase in the number of cell layers. 

 With the low magnification used these details could not, of course, 

 be shown. 



The trachea, ta, is of much smaller caliber tlian the oesophagus, 

 especiallv in its dorso-ventral diameter. While its epithelial lining 

 is not vet appreciably dififerent from that of the oesophagus, its 

 connective tissue wall is much thicker and shows numerous conden- 

 sations, the rudiments of the cartilaginous rings. In the region 

 represented by this figure the connective tissue layers of the trachea 

 and oesophagus are continuous with each other, but cephalad and 

 caudad to this point they are distinct, though sometimes in contact. 

 Several large blood vessels, bz>, on each side of the oesophagus 

 probably represent the carotids and jugulars, but they were not 

 worked out to determine with certainty which they were. 



Eighty-five sections (figure 7, X) caudad to the one under dis- 

 cussion the trachea divides into the two bronchi. These bronchi 

 gradually separate from each other until, at the point at which they 

 open into the lungs, about eighty sections caudad to their point of 

 separation, they lie on either side of the ventral third of the 

 oesophagus. 



Figure 7B represents a section through the plane 480 of figure 7. 

 The section is just cephalad to the heart, and passes through the 

 caudal third of the lungs. In, which have the same appearance as in 

 the preceding figure ; also through the extreme cephalic end of the 

 liver, //. The lungs here much more nearly fill the body cavity than 

 in the preceding figure. The section being- caudad to their openings 

 into the lungs the bronchi do not, of course, show. 



The oesophagus, oe, is here of much less diameter than in the 

 preceding figure, but is still laterally compressed. Its wall is 

 somewhat thicker than in the more cephalic region, the increase 

 being mainly due to the greater thickness of the connective tissue 

 layer, though the epithelium is also slightly thicker because of an 

 increase in the length of the lining cells. Instead of lying almost 

 entirely ventrad to the lungs, as in the preceding figure, the oesoph- 

 agus here lies directly between them. 



Figure 7c represents a section through the plane 627 of figure 7. 

 The plane of the section passes through the opening of the stomach, 

 i', into the duodenum, d. The cross section of the stomach is 

 somewhat larger than that of the oesophagus, but it difl^ers from the 

 more anterior region mainly in the character of its walls. These 

 are much thicker than in the oesophagus ; in the mesoblast which 



